Phase change technology is promising for next generation memory devices. It uses chalcogenide semiconductors for storing states and digital information. The chalcogenide semiconductors, also called phase change materials, have a crystalline state and an amorphous state. In the crystalline state, the phase change materials have low resistivity; while in the amorphous state, they have high resistivity. The resistivity ratios of the phase change material in the amorphous and crystalline states are typically greater than 1000, and thus the phase change memory devices are unlikely to have errors for reading states. The chalcogenide semiconductors are stable at a certain temperature range in both crystalline and amorphous states and can be switched back and forth between the two states by electric pulses. Because of the large resistivity ratio of the crystalline and amorphous states of the phase change material, the resistivities of intermediate states between crystalline and amorphous can also show good enough resolution for multilevel recording. The multilevel recording is another advantage of phase change memory.
Typically, a phase change memory device is formed by placing a phase change material between two electrodes. Write operations, also called programming operations, which apply electric pulses to the memory device, and read operations, which measure the resistance of the phase change memory, are performed through the two electrodes. FIG. 1 illustrates required temperatures for typical programming operations. The temperatures are illustrated as a function of time. A set operation that crystallizes the phase change material is illustrated as line 4. The set pulse needs to heat up the phase change material to a temperature higher than a crystallization temperature Tx, but below a melting temperature Tm, for a time t2 longer than the required crystalline time, for the crystallization to take place. A reset operation that turns the phase change material into an amorphous state is illustrated by line 2. The reset pulse needs to heat up the phase change material to a temperature higher than the melting temperature Tm. The temperature is then quickly dropped below the crystallization temperature Tx during a time period t1, which must be short enough to avoid the crystallization from occurring.
One of the significant challenges that the phase change memory devices face is to reduce the programming current. A commonly used technique for reducing the programming current is to reduce the contact area of the electrical conducting path through the memory device. FIG. 2 illustrates a conventional phase change memory device having reduced contact area. A trench 10 is formed in an insulator 12. A conductive interfacial layer 14 is formed covering the trench 10 and insulator 12. Spacers 16 are formed in trench 10. Phase change material is then deposited into the remainder of the trench to form a phase change layer 18. Since the contact area 17 between the phase change material layer 18 and the interfacial layer is reduced by the spacers 16, the effective resistance is increased and the required programming current is reduced.
Another challenge is to improve the reset speed, which involves quickly dissipating the heat. Typically, the reset speed depends mainly on the intrinsic property of the phase change materials.
FIG. 3 illustrates a prior art phase change memory device having an edge contact. Phase change material layer 26 has a contact region 24 with the edge of a conductive line 20. Since it is easy to form a thin conductive line 20, the contact region 24 can have an area as small 0.004 μm2. Therefore, the required current density is significantly reduced.
The prior art solutions suffer some drawbacks, however. The memory device of the prior art may have an undesirable high current density at an interface area, causing alloying of a phase change material and its neighboring material. It is harder to control programming current density in a manageable range when the contact area is very small, hence the control is inaccurate. Inaccurately controlled current density may cause instability of state change in the reset operation. Therefore, there is a need for alternative solutions.